An Optical Parametric Oscillator (OPO) is a device employing one or more non-linear crystals which when pumped by a laser beam defining a pump wavelength, can generate coherent light at two different and longer wavelengths. In the OPO at least one non-linear crystal (such as BaB2O4, LiB3O5, LiNbO3, KTiOPO4 and others) is placed in an optical resonator. When the pump laser beam is directed to propagate through the crystal, a pair of beams (referred to as the signal beam and the idler beam) is produced. Energy of the photons in the beams is conserved so:
      1          λ      p        =            1              λ        s              +          1              λ        i            
where λp refers to the wavelength of the pump beam, λs refers to the wavelength of the signal beam and λi refers to the wavelength of the idler beam. Typically the shorter wavelength beam is referred to as the signal beam and the longer wavelength beam is referred to as the idler beam. Software is available on the Internet for selecting non-linear crystals and modeling their performance. This software is referred to as SNLO (for “Select Non-Linear Optics”). This software was developed by Dr. Arlee Smith and is available at the web site of AS Photonis with offices in Albuquerque, N. Mex.
The momentum of the photons has to be preserved as presented by the following equation:
            n      p              λ      p        =                    n        s                    λ        s              +                  n        i                    λ        i            
where np, ns, ni, are the refraction indices of the pump, signal, and Idler beams in the crystal material. The momentum equation can be solved only for birefringent crystals in which the index of refraction is not only a function of the wavelength, but it also depends on the polarization orientation of the beam with respect to the optical axis of the crystal. For a given crystal, one can easily calculate the angle between the optical axis of the crystal and the propagation direction of the pump beam that will provide a solution to the above two equations. In practice, by changing the angle between the crystal(s) and the direction of the beams one can select the desired signal wavelength and the corresponding idler.
The generation of the parametric beams (the idler and the signal) in a single path through the crystal(s) is inefficient and only a small fraction of the pump beam is converted. In order to construct an efficient and useful device the crystal(s) are typically placed inside a resonator that is designed to oscillate one or both of the parametric beams inside the cavity, such that it (or they) are amplified in successive passes through the crystal(s). The oscillator components of the OPO are typically comprised of optical elements designed to provide the required feedback for efficient conversion. The principles of OPO are well known and described in many publications on lasers and non-linear optics (for example, A. Yariv, Quantum Electronics, 3rd edition, p. 411. John Wiley & Sons, New York). In many of these OPO's the wavelengths of the signal beam and therefore the idler beam can be tuned over a wide spectral range by varying the orientation of the crystal with respect to the laser beam, by changing the crystal's temperature, or by applying a variable voltage across the crystal. Various tuning ranges can be achieved by properly selecting the laser, the non-linear crystal, and the optical components. Ring oscillators provide high efficiency conversion. A good example is described in U.S. Pat. No. 5,216,598 issued Jan. 4, 1994.
Optical parametric oscillators (OPO's) have been recognized as critical devices for a wide range of applications. In the early stages they were used primarily for research applications and as the designs of these devices have improved they have been incorporated in instruments that are used in commercial applications. In most cases, the wavelength tuning speed, meaning the time it takes to switch from one wavelength to another, is not a major issue. However, for some applications, such as in-vivo medical Imaging and high frame rate hyper-spectral Imaging, fast tuning is critical.